Polar (cataclysmic variable)
Encyclopedia
A Polar is a type of cataclysmic variable binary star
system with a very strong magnetic field
.
In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf
star in the form of an accretion disk. In polar systems, the magnetic field of the white dwarf is too strong for an accretion disk to form. Infalling gas will form an accretion stream, following the dipole
magnetic field lines of the white dwarf.
Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 tesla
s). The star AN Ursae Majoris
has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT).
Polars derive their name from the linearly and circularly polarized light that they produce. Polarization is important to the study of polars, and information about the geometry of the binary star system can be found by studying the polarization.
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system with a very strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
.
In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
star in the form of an accretion disk. In polar systems, the magnetic field of the white dwarf is too strong for an accretion disk to form. Infalling gas will form an accretion stream, following the dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
magnetic field lines of the white dwarf.
Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s). The star AN Ursae Majoris
AN Ursae Majoris
AN Ursae Majoris is a variable star in the constellation Ursa Major. This star has the strongest known magnetic field among cataclysmic variables , with a field strength of 230 million gauss . This object, along with AM Herculis, define a class of objects known as polars....
has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT).
Polars derive their name from the linearly and circularly polarized light that they produce. Polarization is important to the study of polars, and information about the geometry of the binary star system can be found by studying the polarization.
See also
- cataclysmic variable
- variable starVariable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...